-LRB- CNN -RRB- -- Twelve people walked into a Colorado movie theater late one July night , expecting to enjoy a special midnight screening of the Batman film , `` The Dark Knight Rises . ''

They did n't walk out .

Their deaths -- and injuries to dozens of others -- came after a man dressed head to toe in protective tactical gear sprayed the Century 16 theater in Aurora with bullets from an AR-15 rifle , a 12-gauge shotgun and a .40 - caliber pistol , weapons police discovered at the scene .

On Monday , many grisly details of what happened could be presented in a Colorado court at a preliminary hearing for the shooting suspect , James Holmes .

Now 25 , Holmes faces 166 charges , including murder , attempted murder and weapons offenses , tied to the July 20 rampage .

Bits and pieces emerged from police and witnesses shortly after the massacre . One was that Holmes had colored his hair red and told police he was `` the Joker , '' one of Batman 's archenemies , according to a federal law enforcement source with detailed knowledge of the investigation .

But much information has been kept under wraps , especially after Arapahoe County District Judge William Sylvester issued a gag order to prevent defense lawyers , prosecutors and law enforcement agencies from disclosing certain information to the media .

That could change after this week 's hearing , which is expected to last several days .

Prosecutors are expected to call scores of witnesses and outline their evidence in the case . Holmes 's attorneys , meanwhile , are expected to argue he has `` diminished capacity , '' a term that , according to the Colorado Bar Association , relates to a person 's ability or inability `` to make adequately considered decisions '' regarding his or her legal representation because of `` mental impairment or for some other reason . ''

After the hearing concludes , Sylvester will determine whether there is enough evidence for Holmes to stand trial .

University releases e-mails related to Holmes

While officials and news reports have shed light on Holmes , his motive has n't been spelled out .

He was a doctoral student in the neuroscience program at the Anschutz Medical Campus of the University of Colorado , Denver , in Aurora , until he withdrew a month before being arrested outside the bullet-riddled movie theater . Holmes had been a patient of a University of Colorado psychiatrist , according to a court document filed by his lawyers .

Authorities later discovered that his sparsely decorated Aurora apartment was booby-trapped with more than 30 homemade grenades and 10 gallons of gasoline , a law enforcement official who saw video showing its interior told CNN . Authorities intentionally detonated two rigged explosives in order to access the third-floor , one-bedroom apartment .

What spurred him to rig his apartment in such a way and to later fatally shoot strangers is not clear . Academically , Holmes excelled at the University of California , Riverside , according to Chancellor Timothy P. White .

His only brush with the law in Colorado appears to have been a 2011 summons for speeding from Aurora police .

The horror Holmes allegedly unleashed inside the movie theater has been better documented , thanks to accounts from survivors and law enforcement sources .

After the movie started , the suspect went out a rear exit door , propped it open , gathered weapons , then re-entered through the same door , according to a source . He then tossed in a canister before starting to shoot .

Screaming moviegoers scrambled to escape from the gunman , who shot at random as he walked up the theater 's steps , witnesses said .

It was a scene `` straight out of a horror film , '' said Chris Ramos , who was inside the theater .

`` He was just literally shooting everyone , like hunting season , '' Ramos said .

Holmes surrendered without resistance within seven minutes of the first calls from panicked moviegoers reporting the shooting , Aurora Police Chief Dan Oates said .

Meanwhile , local hospitals found themselves overwhelmed with victims , including one who was just 4 months old . Soon thereafter , 12 families heard the horrifying news that their loved ones were dead .

`` This is an act of evil , '' Colorado Gov. John Hickenlooper told CNN .

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The preliminary hearing tied to the July 20 shooting is expected to last several days

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Prosecutors will call witnesses , outline evidence ; 12 were killed in Colorado theater shootings

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The defense is expected to present a `` diminished capacity '' defense

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Suspect James Holmes faces 166 charges , including murder and attempted murder